The invention relates to a circuit assembly for operating a gas probe.
A so-called broad band Lambda probe and a circuit assembly for operating such a broadband Lambda probe is, for example, based on the book publication “Bosch Kraftfahrtechnisches Taschenbuch” (“Bosch Automotive Handbook”), 25. edition, October 2003, page 134. Such a probe embodied as a multilayer ceramic substantially consists of a combination of a conventional concentration probe (Nernst probe), which acts as a galvanic cell, and a limiting-current or “pump” cell. A voltage is externally applied to the pump cell. If the voltage is large enough, a so-called limiting current arises which is proportional to the difference between the oxygen concentrations on both sides of the probe. Oxygen ions are transported—as a function of polarity—with the current. The circuit assembly, which constitutes an electronic regulating circuit, ensures that the concentration probe is always supplied by the pump cell via a very narrow diffusion gap with exactly as much oxygen from the exhaust gas that the state Lambda=1 prevails at said probe. In the case of an excess of air in the exhaust, in the so-called lean range, oxygen is pumped off; whereas in the case of a small residual oxygen content in the exhaust gas, i.e. in the rich range, oxygen is supplied to said concentration probe by reversing the pump voltage. The pump current thereby forms the output signal of the probe.
In the case of such two-cell broadband Lambda probes, a constant nominal (Nernst) voltage is predetermined, which amounts to at least 450 mV. This constant voltage serves as the nominal value for a pump current controller. The pump current controller has the task of controlling the Nernst cell voltage to the desired, constant nominal value by varying the level and polarity of the pump current.
This control of the Nernst voltage to a constant value requires a defined operating point of the probe, an operating point whereat the probe works properly and which is characterized, for example, by a nominal operating temperature. If, for example, said nominal operating temperature is not achieved, a proper operation of the probe is not ensured. When the operating temperature is undershot, oscillations of the pump current controller can then, for example, occur as a result of controlling the Nernst voltage to a constant value.